In various electronic devices, the bias voltage generating circuit is widely used and plays an important role. The main function of a bias voltage generating circuit is to provide a stable bias voltage for the circuits downstream so that they can be operated smoothly. For example, a direct current bias voltage generating circuit which is electrically connected to a transistor provides a stable direct current bias signal so that the transistor can be operated within a working range, a saturation range, a cut-off range or any other operating range as desired.
A conventional bias voltage generating circuit has many drawbacks. It can be easily affected by power fluctuations or variations in the fabrication process and hence, it cannot perform the required functions of a bias voltage. It may even produce faulty signals and may not be capable of maintaining a bias voltage output in a usable range. To further illustrate the drawbacks of a conventional bias voltage generating circuit, an example which is frequently used in a reference voltage generating circuit is described below.
Referring initially to FIG. 1 which is a circuit diagram of a conventional bias voltage generating circuit that is used in a reference bias voltage generating circuit. The circuit includes a power generating device 1, a reference voltage generating device 2, and a bias voltage generating circuit 3. The power generating device is used to provide a power source V.sub.DD for the reference voltage generating circuit 2 and the bias voltage generating circuit 3. The bias voltage generating circuit 3 is a resistor type bias voltage generating circuit which includes bias resistors R1 and R2.
In FIG. 1, the bias voltage resistors R1, R2 and the equivalent resistance of the NMOS transistor (labeled as Q) enable a voltage division of power source V.sub.DD. At the series connection point (labeled as P) of the bias resistors R1 and R2, a bias voltage output V.sub.bias1 is generated for use by the reference voltage generating circuit 2. However, it is frequently affected by noise signals or other signals in the circuit such that it fluctuates. Consequently, a reliable bias voltage cannot be generated by the properly matched bias voltage resistors R1 and R2. In addition, although the NMOS transistor Q is used as an on/off switch (controlled by the Vcontrol signal), the equivalent resistance of the NMOS transistor Q fluctuates due to variations in the power source V.sub.DD. It is therefore difficult to design a circuit to produce a desirable bias voltage. Furthermore, the bias voltage generated by a conventional bias voltage generating circuit varies greatly with variations in the power source V.sub.DD and drifts easily from a suitable working range.
Moreover, as shown in FIG. 1, a parallel connection between an input resistor Rin at point P of the reference voltage generating circuit 2 and the bias resistor R2 affects the value of the bias voltage V.sub.bias1. Due to the difficulties in controlling manufacturing parameters and the variations in manufacturing time and environment, the characteristics of the components of the reference voltage generating circuit 2 and the bias resistors R1 and R2 can change. Such change can in turn change the input resistor Rin which is closely tied to the components in the reference voltage generating circuit 2, the bias resistors R1 and R2 and the equivalent resistance of the NMOS transistor Q. A drifting in the bias voltage V.sub.bias1 can thus occur. The conventional bias voltage generating circuit easily drifts away from its originally designed working range due to changes occurred in the power source and in the component characteristics. It is not capable of producing a reliable bias voltage signal and moreover, it causes other circuits in the device to generate faulty signals and abnormal reactions.
It is therefore an object of the present invention to provide an intelligent bias voltage generating circuit which is capable of producing a reliable bias voltage that is not affected by fluctuations in the power source.
It is another object of the present invention to provide an intelligent bias voltage generating circuit which is capable of producing a reliable bias voltage that is not affected by changes in the component characteristics.